Abstract
Synthesis and characterization of long wavelength visible-light absorption Cu-doped TiO2 nanomaterials with well-controlled properties such as size, composition, morphology, and crystal phase have been demonstrated in a single-step flame aerosol reactor. This has been feasible by a detailed understanding of the formation and growth of nanoparticles in the high-temperature flame region. The important process parameters controlled were: molar feed ratios of precursors, temperature, and residence time in the high-temperature flame region. The ability to vary the crystal phase of the doped nanomaterials while keeping the primary particle size constant has been demonstrated. Results indicate that increasing the copper dopant concentration promotes an anatase to rutile phase transformation, decreased crystalline nature and primary particle size, and better suspension stability. Annealing the Cu-doped TiO2 nanoparticles increased the crystalline nature and changed the morphology from spherical to hexagonal structure. Measurements indicate a band gap narrowing by 0.8 eV (2.51 eV) was achieved at 15-wt.% copper dopant concentration compared to pristine TiO2 (3.31 eV) synthesized under the same flame conditions. The change in the crystal phase, size, and band gap is attributed to replacement of titanium atoms by copper atoms in the TiO2 crystal.
Highlights
Nanosized TiO2 has been widely used because of its stability in aqueous environments and low production cost
For synthesizing Cu-doped TiO2 particles, both the TTIP and copper nitrate precursor are fed to the high-temperature flame
Norris et al [27] proposed a process called self-purification by which dopants diffuse from inside to the surface sites of TiO2 nanocrystals. This change in particle size with doping concentration is fundamentally a very important phenomenon for electronic structure modification. These results indicate that the particle size of the Cu-doped TiO2 can be controlled by manipulating the dopant concentration in addition to the methods demonstrated by other researchers by controlling the precursor feed concentration and residence time of the particle in the high-temperature flame [26,32]
Summary
Nanosized TiO2 has been widely used because of its stability in aqueous environments and low production cost. To shift the absorption range to the visible spectrum, various approaches have been pursued in the past involving size optimization [1], compositional variation to make sub-oxides [2], surface modification [3], and doping [4,5,6] to modify the TiO2 structure Among these methods, tailoring the band structures by incorporating a dopant into the host nanomaterial is a promising approach [6,7,8]. The role of key process parameters such as molar feed ratio of precursors and dopant concentration on TiO2 nanomaterial properties such as size, composition, crystallinity, stability in suspension, and morphology are thoroughly investigated. The copper dopant concentration was varied from 0 to 15 wt.% to process Cu-doped TiO2
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